4 comments
It's like why the first (hard) drive letter is C.
Nowadays UD0 UD1 UD2 are in the SDM and APM.<p>We also got UDB (D6), the one-byte variant that arrived with x86-64 for 64-bit mode.<p>And we have always had UDW (FF FF), aka group #5 (1st FF) with a modrm byte of mod=11b r/m=111b (/7) reg=111b (2nd FF) -- that one matters for memory with all bits set to 1, or for buses terminated to all 1 when no device claims an access.
Is this just his speculation? Or is there evidence for it?
I'm not much of an x86 person but on other architectures you can raise software interrupts/exceptions. Does x86 not have this or did those facilities not cover enough use cases?
It's basically a convention. The alternative is to raise interrupts of course, but that might be application specific, or use other invalid instructions than the designated one, but they might work differently on other processor types.
x86 has...<p>INT Ib
INT1
INT3
INTO
BOUND